System and method for the manufacture of semiconductor devices by the implantation of carbon clusters

ABSTRACT

A process is disclosed which incorporates implantation of a carbon cluster into a substrate to improve the characteristics of transistor junctions when the substrates are doped with Boron and Phosphorous in the manufacturing of PMOS transistor structures in integrated circuits. There are two processes which result from this novel approach: (1) diffusion control for USJ formation; and (2) high dose carbon implantation for stress engineering. Diffusion control for USJ formation is demonstrated in conjunction with a boron or shallow boron cluster implant of the source/drain structures in PMOS. More particularly, first, a cluster carbon ion, such as C 16 H x   + , is implanted into the source/drain region at approximately the same dose as the subsequent boron implant; followed by a shallow boron, boron cluster, phosphorous or phosphorous cluster ion implant to form the source/drain extensions, preferably using a borohydride cluster, such as B 18 H x   +  or B 10 H x   + . Upon subsequent annealing and activation, the boron diffusion is reduced, due to the gettering of interstitial defects by the carbon atoms.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/634,565, filed on Dec. 6, 2006, now U.S. Pat. No. 7,666,771, which,in turn, claims priority to and the benefit of U.S. Provisional PatentApplication No. 60/748,797, filed on Dec. 9, 2005, hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of semiconductor manufacturingand more particularly to a method for implantation of carbon-containingclusters into a substrate for stress engineering and diffusion controlto improve the characteristics of transistor junctions when dopingsubstrates with boron, arsenic and phosphorous in the manufacturing ofPMOS transistor structures in integrated circuits.

2. Description of the Prior Art

The Ion Implantation Process

The fabrication of semiconductor devices involves, in part, theformation of transistor structures within a silicon substrate by ionimplantation. As disclosed by Sferlazzo et. al. in U.S. Pat. No.5,497,006, ion implantation equipment includes an ion source whichcreates a stream of ions containing a desired dopant species, a beamline which extracts and accelerates the ions from the ion source bymeans of an extraction electrode, and forms and focuses the ion streaminto an ion beam having a well-defined energy or velocity, an ionfiltration system which selects the ion of interest, since there may bedifferent species of ions present within the ion beam, and a processchamber which houses the silicon substrate upon which the ion beamimpinges; the ion beam penetrating a well-defined distance into thesubstrate. Transistor structures are created by passing the ion beamthrough a mask formed directly on the substrate surface, the mask beingconfigured so that only discrete portions of the substrate are exposedto the ion beam. Where dopant ions penetrate into the silicon substrate,the substrate's electrical characteristics are locally modified,creating source, drain and gate structures by the introduction ofelectrical carriers: such as, holes by p-type dopants, such as boron orindium, and electrons by n-type dopants, such as phosphorus or arsenic,for example.

A recent development in semiconductor processing is the incorporation ofmechanical stress to enhance transistor performance. This stress isgenerated by including atoms of elements other than silicon into alattice structure. The successful process to date has been theincorporation of Ge atoms into the source and drain regions of a PMOStransistor. Inclusion of Ge atoms into a silicon substrate forms a SiGealloy, which has a compatible lattice structure with the Si lattice.However, the Ge atoms are larger than the Si atoms, resulting in alarger lattice constant for the SiGe alloy, which can be controlled bythe amount of Ge included. By forming this alloy material in the sourceand drain region of a PMOS transistor, the larger lattice therein placesthe channel region under compressive stress, which enhances the holemobility and increases the performance of the PMOS transistor. Theinclusion of Ge atoms only works for PMOS transistors becausecompressive stress is detrimental to the electron mobility and degradesthe performance of an NMOS transistor.

Prior Art Ion Sources

Traditionally, Bernas-type ion sources have been used in ionimplantation equipment. Such ion sources are known to break downdopant-bearing feed gases, such as BF₃, AsH₃ or PH₃, for example, intotheir atomic or monomer constituents, producing the following ions incopious amounts: B⁺, As⁺ and P⁺. Bernas-type ion sources are known ashot plasma or arc discharge sources and typically incorporate anelectron emitter, either a naked filament cathode or anindirectly-heated cathode. This type of source generates a plasma thatis confined by a magnetic field. Recently, cluster implantation ionsources have been introduced into the equipment market. These ionsources are unlike the Bernas-style sources in that they have beendesigned to produce “clusters”, or conglomerates of dopant atoms inmolecular form, e.g., ions of the form As_(n) ⁺, P_(n) ⁺, or B_(n)H_(m)⁺, where n and m are integers, and 2≦n≦18. Such ionized clusters can beimplanted much closer to the surface of the silicon substrate and athigher doses relative to their monomer (n=1) counterparts, and aretherefore of great interest for forming ultra-shallow p-n transistorjunctions, for example in transistor devices of the 65 nm, 45 nm, or 32nm generations. These cluster sources preserve the parent molecules ofthe feed gases and vapors introduced into the ion source. The mostsuccessful of these have used electron-impact ionization, and do notproduce dense plasmas, but rather generate low ion densities at least100 times smaller than produced by conventional Bernas sources. Forexample, the method of cluster implantation and cluster ion sources hasbeen described by Horsky et al. in U.S. Pat. No. 6,452,338 and U.S. Pat.No. 6,686,595 hereby incorporated by reference. The use of B₁₈H₂₂ as animplant material for ion implantation of B₁₈H_(x) ⁺ in making PMOSdevices is disclosed in Horsky et al., in pending U.S. patentapplication Ser. No. 10/251,491, published as US Patent ApplicationPublication No. US 2004/0002202 A1, hereby incorporated by reference.

Background: USJ Challenges

As device technology continues to scale in all dimensions, it becomesincreasingly difficult to form the p-type Ultra Shallow Junctions, orUSJ, necessary for the proper formation of the PMOS transistor. The mostchallenging feature of the PMOS transistor is the Source/DrainExtension, or SDE, which must be the shallowest junction in thetransistor to be effective. For 65 nm node technology (as defined in theInternational Technology Roadmap for Semiconductors, or ITRS), it isrequired that the PMOS SDE be around 15-25 nm deep, while 45 nmtechnology will require PMOS SDE junction depths of 8-20 nm. There aretwo principal means of controlling the junction depth: (1) controllingthe initial placement of the boron dopants, and (2) controlling theirsubsequent movement during activation. The dopants move whenever theyexperience high temperature, such as during the implant anneal andactivation process. The initial placement of the dopant is determined bythe implant energy of the dopant ion. Both of these means havehistorically been used to scale the vertical dimension of the PMOS SDEas the technology scales to smaller gate lengths. The principal means ofreducing PMOS SDE junction depth in recent generations has been byreducing the annealing time during the activation step, which reducesdopant diffusion and thereby results in the formation of a shallowerjunction. The implant energy has also been reduced to make the initialdopant placement shallower, i.e., closer to the silicon surface, butsince implanter beam currents are reduced at lower beam energy, andsubstantially so at the sub-keV boron energies required for boronimplantation for sub-90 nm feature sizes, this significantly reduces theproductivity (wafer throughput) of the implant tool, and so is not anattractive means to reduce junction depth.

Diffusion Control

Diffusion is a natural result of the need to activate the implantedboron, that is, the implanted wafer must be annealed at high temperaturefor the implanted boron to become electrically active in the silicon.Whenever silicon containing boron is exposed to high temperatures, theboron will move, or diffuse, from regions of high concentration toregions of lower concentration. This boron movement challenges theformation of very shallow junctions. The major trend in annealdevelopment has been the reduction of annealing time, which manages thenet diffusion. Modern wafer processing incorporates a “spike” annealwhich quickly ramps to a high temperature (1000-1100 C) and down again.This technique has been very effective in reducing diffusion andproviding a production worthy process solution. However, for sub-90 nmnode technology even less boron diffusion is required; the ramp ratelimit of spike annealing systems (up to 150 C/sec) has been reached. Thenext step in reducing thermal budget this trend will likely use the“millisecond” technologies of either “flash annealing” or laser thermalprocessing. Both of these technologies are still immature and facesignificant challenges in providing production worthy solutions. Also,as thermal budgets are reduced to very small values by these ultra-fastannealing approaches, the activation process itself is affected. Forexample, higher sheet resistances are achieved and the spatialnon-uniformity of the sheet resistance, or Rs, is higher than achievedby spike anneals.

Co-Implantation

One alternative to the continued reduction of annealing time is theintroduction of other impurities which are known to hinder the diffusionof boron, potentially resulting in a shallower junction at the samethermal budget. For example, it is well known that F, historicallyintroduced during a BF₂ ⁺ implantation step, can reduce boron diffusion.Thus, junctions formed by a BF₂ ⁺ implant are usually shallower than theequivalent B⁺ implant when the same annealing process is used. However,since the F is introduced in the same implantation process as the boron,i.e., as part of the molecular species BF₃, the as-implanted F depthprofile that results from BF₂ ⁺ implant is not optimized for thereduction of B diffusion; this makes BF₂ implantation less attractive asjunction depths are reduced further.

Another alternative is the introduction of carbon, which is also knownto inhibit diffusion. See for example, E. J. Gonad, S. B. Fetch, H.Graoui, D. Kirkwood, B. J. Pawlak, P. P. Absil, S. Sevri, T. Janssensand W. Vandervorst, “Co-Implantation with Conventional Spike AnnealSolutions for 45 nm Ultra-Shallow Junction Formation”, Proceedings ofthe Eight International Workshop on: Fabrication, Characterization andModelling of Ultra-Shallow Doping Profiles in Semiconductors, June 2005,p. 327; N. Cowern, B. Colombeau, J. Graoui, and M. Foad, “ComputationalModeling of Co-iimplanted Carbon for 65 nm Node USJ Formation, ibid, p.300.; S. Rizk, Y. M. Haddara and A. Sibaja-Hernandez, “Modeling theSuppression of Boron Diffusion in Si/SiGe Due to Carbon Incorporation,ibid, p. 315; L. S. Robertson, R. Brindos, and K. S. Jones, “The effectof impurities and activation of ion implanted boron in silicon”, Mat.Res. Soc. Symp. Vol. 610, pp. B5.8.1-B5.8.6 (2000); Mark E. Law,Michelle D. Griglione, and Misty Northridge, “Influence of Carbon on theDiffusion of Interstitials and Boron in Silicon”, ibid, pp.B7.4.1-B7.4.5; E. J. H. CoIlart et al., “Co-implantation withconventional spike anneal solutions for 45 nm ultra-shallow junctionformation”, Internal Conference on Ultra-Shallow Junctions, USJ2005,June 5-8, Florida, USA (2005); P. A. Stolk, H.-J. Gossmann, D. J.Eaglesham, D. J. Jacobson, H. S. Luftman, and J. M. Poate,“Understanding and controlling transient enhanced dopant diffusion insilicon”, Mat. Res. Soc. Symp. Proc. Vol. 354, pp. 307-318 (1995); M.Ueda, H. Reuther, R. Gunzel, A. F. Beloto, E. Abramof, and L. A. Bemi,“High dose nitrogen and carbon shallow implantation in Si by plasmaimmersion ion implantation”, Nuclear Instruments and Methods in PhysicsResearch B175-07 (2001) pp. 715-720; Jörg K. N. Lindner, “Ion beamsynthesis of buried SiC layers in silicon: Basic physical processes”,Nuclear Instruments and Methods in Physics Research B 178 (2001) pp.44-54; J. K. N. Lindner, W. Reiber and B. Stritzker, “Mechanisms of SiCFormation in the Ion Beam Synthesis of 3C—SiC Layers in Silicon”,Materials Science Forum Vols. 264-268 (1998) pp. 215-218; M. Ueda etal., “High dose nitrogen and carbon shallow implantation in Si by plasmaimmersion ion implantation”, Nuclear Instruments and Methods in PhysicsResearch B 175-177 (2001). pp. 715-720; Kah-Wee Ang et al., “Thin bodysilicon-on-insulator N-MOSFET with silicon-carbon source/drain regionsfor performance enhancement”, IEDM Workshop, Washington, D.C., December,2005; Masahiro Deguchi, Akihisa Yoshida, and Masatoshi Kitagawa, “B—SiCformation by low-energy ion-doping technique”, Japanese Journal ofApplied Physics Vol. 29, No. 8, August, 1990, pp. L 1493-L 1496, allhereby incorporated by reference.

Stress Engineering and Carrier Mobility Enhancement

One of the newer trends in silicon processing is called stressengineering, whereby structures are created which purposefully putactive structures under mechanical stress due to lattice mismatch. Theprimary goal of such effort is to put the channel region of a MOStransistor under stress to enhance the carrier mobility. PMOStransistors benefit from compressive stress, and NMOS transistorsbenefit from tensile stress. The leading process for creatingcompressive stress is the incorporation of Ge into the silicon lattice,and forming a SiGe alloy. Ge is useful in this role because it is alarger atom than the silicon and the incorporation of Ge forces thelattice to expand, thereby placing adjacent regions under stress. Themost public use of this concept is the Intel 90 nm technology wherebythe PMOS source/drain regions are epitaxially grown SiGe alloystructures which place the channel region in compressive stress, therebyenhancing the performance of the PMOS transistor and the overallcircuit.

Stress Engineering for Sub-65 nm Logic and Memory Applications

The 65 nm technology node requires further reduction of the SDE junctiondepth to produce appropriate transistor structures. Starting with thisnode, the PMOS SDE becomes very challenging from both a process controland productivity perspective. The energy of the boron implant must bereduced to 500 eV boron energy or less, where productivity is markedlyreduced. Also, the annealing requirements are such that advanced,low-thermal budget spike anneals are required and even new, alternativeannealing technologies must be considered. Due to the difficulty ofachieving these process goals, alternative processes must be considered.The introduction of stress engineering to produce a higher performancetransistor can alleviate the immediate need to reduce the gate length inorder to enhance speed and drive current. The concept is that producinga transistor where the active channel is permanently under stress allowsthe formation of higher performance transistors. The mechanism for thisadvancement is that the stress modifies the carrier mobility and therebydirectly improves the transistor drive current and therefore the speedof the circuit. The stress engineering required is not simple, since theNMOS transistor performance improves with tensile stress while the PMOStransistor performance improves with compressive stress. For example,Intel has integrated stress engineering at the 90 nm node by theintroduction of SiGe source/drain technology which places the PMOSchannel under compressive stress and thereby improves the performance ofthe PMOS and the use of a selective silicon nitride overlayer whichplaces the NMOS transistor in tensile stress, enhancing NMOSperformance. The combination of these stress engineering technologiesresults in significant performance enhancement for the circuit overall.To be competitive with these processes, some analysts believe thatstress engineering will be required for all high performance productsstarting at the 65 nm node.

So-Called Diffusionless Anneals

To maximize the benefit of reducing annealing time, the current trend isto continuously reduce the time of the annealing process (the thermalbudget) and so reduce the boron diffusion. The peak temperature remainshigh (typically higher than 1000 C) in order to achieve high activationfor the dopant atoms. Current production processes use a “spike anneal”which involves a ramp to temperature, zero time at max temperature, andthen a ramp down in temperature back to room temp. Typical values of theramp rates are from 100 C/s to 1000 C/s, with ramp rate to temperaturedriven by choice of heating technology. Cool down ramp rate is generallylimited to less than 400 C/s unless active cooling is employed. Newsystems are constantly being developed which increase ramp rate andthereby reduce total time at high temperature.

The limit of this approach, now under development, is genericallylabeled “diffusionless anneal”. There are two technologies underdevelopment which might provide such a process option: “flash” annealand “laser thermal anneal”. Both of these technologies have thepotential to provide an anneal process with adequate activation (peaktemperatures typically 1300 C) and very little boron diffusion. Thesetechnologies are also called “millisecond anneals” because the siliconexperiences the high temperature required for activation for only amillisecond or less. While this process alternative is Very attractive,the technologies required for such a process are radically differentthan the annealing technologies currently in production. These tools arenew and do not yet meet the requirements for large scale production. Itis uncertain whether diffusionless anneals will provide robustmanufacturing solutions in the near future.

Carbon Implantation (Gettering Implants)

Carbon implantation has been used as a method of gettering defects orcontaminants for some time. See, for example, the Stolk et al and theUeda et al references above. Since defects have been shown to drivetransient enhanced diffusion of B and P in silicon, trappinginterstitial defects has been seen as a candidate method for limitingdiffusion. Conventional processes use either CO₂ or CO gas source to aconventional plasma ion source. Beams of C⁺ are generated andimplantation can be performed with a commercial ion implantation system.The use of CO₂ or CO gas degrades the service lifetime of conventionalplasma sources due to oxidation effects and also carbon tracking ofinsulators found in the sources.

One prior application of carbon implants is to provide gettering ofmetallic impurities by implanting high energy (MeV) carbon deep into thesilicon, away from the transistor structure. In silicon, any metal atomspresent can degrade the electrical performance of active structures,principally by increasing leakage. There has been much research ofmethods of removing metallic impurities from the active device regions.One approach which has been used is the implantation of carbon into thesilicon away from the active devices. Since carbon in silicon acts as animpurity trap, any metallic atoms that interact with the carbon willstay in that location; even when high temperatures are experienced. Thismechanism is called gettering and carbon implants are one of the optionsfor gettering.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to a process which incorporatesimplantation of a carbon cluster into a substrate to improve thecharacteristics of transistor junctions when the substrates are dopedwith boron, arsenic and phosphorous in the manufacturing of PMOStransistor structures in integrated circuits. There are two processeswhich result from this novel approach: (1) diffusion control for USJformation; and (2) high dose carbon implantation for stress engineering.Diffusion control for USJ formation is demonstrated in conjunction witha boron or shallow boron cluster implant of the source/drain structuresin PMOS. More particularly, a cluster carbon ion, such as C₁₆H_(x) ⁺, isimplanted into the source/drain region at approximately the same dose asthe subsequent boron implant; followed by a shallow boron implant toform the source/drain extensions, preferably using a borohydridecluster, such as B₁₈H_(x) ⁺ or B₁₀H_(x) ⁺. Upon subsequent annealing andactivation, the boron diffusion is reduced, due to the gettering ofinterstitial defects by the carbon atoms. The Stolk et al., andRobertson et al references, mentioned above, assert that transientenhanced diffusion of boron is mediated by interstitial defects in thesilicon lattice.

There is a secondary benefit to the process in accordance with thepresent invention that is of great economic value; namely, amorphizationof the silicon by the carbon cluster implant and the subsequent boroncluster implant. In particular, it is known in the art that implantationof large boron clusters amorphize the crystalline silicon lattice,resulting in a large reduction in ion channeling. Channeling is known tosignificantly increase the junction depth of a dopant implant therebymaking USJ formation difficult. Channeling is typically controlled byfirst conducting a damage implant, for example by Ge⁺ atoms, toamorphise the silicon prior to forming the PMOS source/drain extensionsby implanting boron. The Ge⁺ implant is expensive to perform, and hasalso been shown to create defects at its end-of-range which increasesleakage currents in the USJ transistors, in certain cases by severalorders of magnitude. Thus, replacing the conventional boron implant by aboron cluster, such as B₁₈H_(X) ⁺, obviates the need for the Ge implantin many cases, and does not generate the defect structures associatedwith leakage. This is of great economic value since is removes a costlyimplant from the process flow, increases yield, and reduces leakagecurrent in the integrated circuits. Unfortunately, the boron clusterimplant can never entirely eliminate channeling, since before thecritical dose for amorphization is reached, a fraction of the boroncluster implant has been performed, and has contributed to a channelingtail. For example, an 80% reduction in channeling has been demonstratedby the implantation of B₁₈H_(X) ⁺ ions to form the source/drainextensions, but to achieve the most shallow junctions, a damage implantis still required. If, however, according to the process in accordancewith the present invention, a carbon cluster is implanted followed by animplant of either boron clusters or monomer boron, the silicon has beenfully amorphized by the carbon implant, so channeling can be entirelyeliminated.

Finally, if thermally-driven boron diffusion can be minimized by theappropriate carbon co-implant, the need for diffusionless anneals can beaverted. For example, FIGS. 3-7 suggest that a standard, commerciallyviable spike anneal should be adequate to form. USJ structures. Thiswould be of great economic benefit to the chip making industry, notrequiring next-generation exotic annealing technologies.

Thus, the benefit of this method is:

-   -   Amorphization of the silicon prior to the PMOS USJ formation;    -   Elimination of the channeling tail in the subsequent boron or        boron cluster implant, resulting in shallowest as-implanted        profiles;    -   Dramatically reduced boron diffusion during the annealing step;    -   Elimination of the pre-amorphization, or damage, implant;    -   Relaxing the thermal budget requirements in the annealing        process, enabling USJ formation with spike anneals.

In addition, when very shallow carbon implants are required, the use ofa carbon cluster by the process in accordance with the present inventionincreases the effective dose rate (and hence the wafer throughput)relative to performing monomer carbon implants, providing a similarincrease to throughput achieved by substituting boron clusters formonomer boron.

Phosphorus Diffusion Control

The preceeding discussion focused on the use of carbon to inhibitdiffusion of boron implants for the formation of the PMOS transistor.Likewise, the same discussion applies to the formation of the NMOStransistor if phosphorus is used as the dopant atom. Conventionally, anarsenic implant is used to form the NMOS SDE structure, and Arsenicexhibits slow diffusion in silicon, so no additional means are necessaryto form the appropriate structure. However, there are issues which mightpreclude the use of Arsenic for the SDE in advanced technologies. Inthese cases, phosphorus may become a viable alternative, but phosphorusexhibits fast diffusion in silicon and so diffusion inhibition by carbonbecomes an attractive process. An example of results showing carbon tobe useful for diffusion control with phosphorus, implants is A.Vanderpool, A. Budrevich and M. Taylor, “Control of Phosphorus TransientEnhanced Diffusion using Co-Implantation, Proceedings of the 16^(th)International Conference on Ion Implantation Technology, June 2006, p.41.

The use of ionized clusters of carbon and/or boron requires a novel ionsource, for example, as disclosed in U.S. Pat. No. 6,686,595, herebyincorporated by reference A vapor of, preferably, a hydrocarbon isintroduced into the ion source. The ion source ionizes the moleculewithout dissociation. The extraction system then extracts an ion beam ofthe ionized carbon molecule which is then transported down the beam lineof a conventional ion implanter to impact the silicon wafer. AClusterIon® ion source developed by SemEquip, as disclosed in the '595patent, for example, may be used for the implantation of ClusterBoron®molecules, such as B₁₈H₂₂,

The same technology has now been demonstrated to work with largehydrocarbon feed materials. For example, the following hydrocarbons maypotentially be used:

-   -   2,6 diisopropylnaphthalene (C₁₆H₂₀)    -   N-octadene (C₁₈H₃₈)    -   P-Terphenyl (C₁₈H₁₄)    -   Bibenzyl (C₁₄H₁₄)    -   1-phenylnaphthalene (C₁₆H₁₀)    -   Flouranthene (C₁₆H₁₀)

Flouranthene is the material used to generate much of the data includedin this disclosure. In general, any hydrocarbon with a chemical formulaof the form CnHy, where n≧4 and y≦0 will provide the above feature andbenefits, namely, increasing the effective carbon dose rate into thesilicon, and providing varying degrees of amorphization, in all casesbeing more beneficial than a monomer carbon implant.

The C₁₆H₁₀ vaporizes at a temperature of 100 C, well suited to the novelion source, and similar to the vaporization temperature of B₁₈H₂₂, asdisclosed in pending U.S. patent application Ser. No. 10/251,491,mentioned above. A beam current of 0.5 mA enabled the equivalent of 8 mAof carbon on the wafer, at very low energy (about 1 keV per carbonatom). Using a ClusterIon® source, beam currents of >1 mA are easilyrealized.

Monomer Vs Cluster Carbon Implantation

Carbon implantation has been available essentially since theintroduction of commercial ion implantation systems for semiconductormanufacturing, which started in the mid-1970s. The availableimplantation technology performs the implant one atom at a time,regardless of the feed material used. This occurs because conventionalion source technology uses an intense plasma to ionize the material, andthe plasma breaks apart molecules into their component atoms. For mostapplications, this works well. The issue with the conventionaltechnology is that the implantation by single atoms becomes veryinefficient when the energy of the ions must be low (e.g., 1 or 2 keV)in order to provide for shallow implant. Conventional systems cannotproduce high currents at low extraction energy and so the implantationprocess has very low productivity. This is exactly the problem with lowenergy boron implantation, and the physics are the same for low energycarbon implants. The invention described here enables the implantationof carbon clusters at low energies with very high productivity. Byimplanting carbon-containing molecules instead of individual atoms, thephysics of low energy implantation are significantly modified. Theextraction energy is much higher, since the molecule needs the processenergy for each carbon atom, which makes the extraction system able tooperate efficiently and generate high beam currents.

Plasma Doping with Clusters

An alternative approach to beam line ion implantation for the doping ofsemiconductors is so-called “plasma immersion”. This technique is knownby several other names in the semiconductor industry; such as RAD(PLAsma Doping), PPLAD (Pulsed PLAsma Doping, and PI³ (Plasma ImmersionIon Implantation). Plasma doping is well known in art. See for example:A. Renau and J. T. Scheuer, “Comparison of Plasma Doping and BeamlineTechnologies for Low energy Ion Implantation”, IEEE Proceedings of the2002 14th International Conference on Ion Implantation Technology, Taos,N. Mex., USA, 22-27 Sep. 2002, pp. 151-156; R. B. Liebert, S. R.Walther, S. B. Fetch, Z. Fang, B. Pedersen, D. Hacker, “Plasma DopingSystem for 200 mm and 300 mm Wafers, “Proceedings, 13th Int. Conf. IonImplant. Tech., IEEE, 2000, pp. 472-475, as well as U.S. Pat. Nos.5,354,381; 5,558,718; and 6,207,005, all hereby incorporated byreference

Doping using these techniques requires striking a plasma in a largevacuum vessel that has been evacuated and then backfilled with a gascontaining the dopant of choice such as boron triflouride, diborane,arsine, or phosphine. The plasma by definition has positive ions,negative ions and electrons in it. The target is then biased negativelythus causing the positive ions in the plasma to be accelerated towardthe target. The energy of the ions is described by the equation U=QV,where U is the kinetic energy of the ions, Q is the charge on the ion,and V is the bias on the wafer. With this technique there is no massanalysis. All positive ions in the plasma are accelerated and implantedinto the wafer. Therefore extremely clean plasma must be generated. Withthis technique of doping a vapor of boron clusters, such as B₁₈H₂₂, orarsenic or phosphorus clusters of the form As_(n)H_(x) and P_(n)H_(x)(where n is an integer and x≦0) can be introduced into the vessel and aplasma ignited, followed by the application of a negative bias on thewafer. The bias can be constant in time, time-varying, or pulsed. Theuse of these clusters will be beneficial since the ratio of dopant atomsto hydrogen (e.g., using B₁₈H₂₂ versus B₂H₆ and, for example As₄H_(x)versus AsH₃) is greater for hydride clusters than for simple hydrides,and also the dose rates can be much higher when using clusters.

Plasma doping can also be used for carbon cluster implants. Moreparticularly, an analogous argument can be made for using carbonclusters of the form C_(n)H_(x) to dope and preamorphize silicon wafersin a plasma doping system, prior to performing the conductive implantsusing As, B or P. In a plasma doping system, dose can be parametricallycontrolled by knowing the relationship between pressure of the vapor inthe vessel, the temperature, the magnitude of the biasing and the dutycycle of the bias voltage and the ion arrival rate on the target. It isalso possible to directly measure the current on the target. As withbeam line implantation, using C₁₆H_(x) ⁺ ions would yield an 16 timesenhancement in dose rate and 18 times higher accelerating voltagesrequired than for CH_(x) ⁺ ions, for example.

The Importance of Implantation Depth

As the junction requirements become shallower, the boron implant energymust be reduced. Likewise, the carbon implant energy required toeffectively reduce diffusion must also be reduced. The use of the carboncluster or molecule enables very high productivity at very low energies,which is required for advanced technologies. Another critical concerndriving the carbon implant process to lower energies is the control ofleakage current. Carbon at high concentrations in a junction region isknown to endanger low leakage operation. Since most of the activejunction is deeper than the source/drain extension junction, carbon athigher energies gets placed exactly in the region of highest leakagerisk. One approach to managing this issue is to keep the carbon clusteras shallow as possible, and minimize the dose. In this way, anyenhancement of the junction leakage current is minimized.

Pre-Amorphization

A still further advantage supporting the use of carbon implantation withclusters for diffusion control is pre-amorphization. In order to makethe boron implant profile as shallow as possible, ion channeling must beavoided. Since the geometry of the transistor formation requires thatthe implant be performed at normal incidence, tilted implant cannot beused to prevent ion channeling. The conventional approach is to useanother implant to destroy the silicon lattice structure prior toperforming the boron implant, and this is generally referred to as a“PAI” or pre-amorphization implant. The implant is generally performedusing Ge+ ions since they have high mass (and thereby create anamorphous silicon layer at relatively low dose) and the Ge isincorporated into the silicon without much impact on the electricalproperties of the devices. However, the Ge implant is difficult andexpensive and creates another damage network which risks creatingjunction leakage. In the proposed technology, the carbon cluster implantprovides a PAI benefit because it is a cluster of significant mass. Thecarbon implant must be performed before the boron implant to havemaximum effect, and the use of the cluster thereby also performs the PAIfunction. A significant reduction of implanted boron depth profileresults with this use of the carbon cluster., in accordance with animportant aspect of the invention.

Process Flow

There is information in the literature that indicates the carbon implantmust be performed before the boron implant to be most effective atcontrolling diffusion. This means that the carbon implant occurs aftergate stack formation and patterning and before the boron SDE implant.The masking operation required is identical to the boron implant, so noadditional or modified lithography is required. In fact, the carboncluster and boron or ClusterBoron implants can be performed in a chain,without removing the wafers from the ion implanter; this is ofsignificant economic benefit on batch tools.

Stress Engineering

It has been shown that carbon incorporated into the source/drain regionsof the transistor in silicon can form Si_(x)C_(y) material whichprovides a lattice mismatch to pure silicon and therefore mechanicallystresses the transistor channel, increasing carrier mobility, asdiscussed in the Ang, et al. reference, mentioned above. The Si_(x)C_(y)material has a smaller lattice than the silicon, so this materialcreates tensile stress in the channel which is useful for improving themobility of NMOS transistors. As such, in accordance with an importantaspect of the invention, carbon cluster implantation, such as withC₁₆H₁₀ ⁺, is used to perform high dose implants as the means ofconverting silicon into Si_(x)C_(y) selectively in the source/drainregions of an NMOS transistor. At a given ion current, the use of acluster like C₁₆H₁₀ multiplies the carbon dose by a factor of sixteen,and enables ultra shallow implants at high dose

An additional benefit of forming the Si_(x)C_(y) material byimplantation is the control afforded by the implantation equipment. Ionimplantation is a successful process in semiconductor manufacturing ingeneral because the accuracy and control of the equipment far surpassesthe capability of other forms of semiconductor processing equipment. Inparticular, for the proposed application, the in-depth profile of carbonconcentration can be managed in detail by the control of the implantenergy and dose. Indeed, one could envision a sequence of implant stepswith various doses and energies to contour the carbon profile to anyprofile desired. Since it is unclear what detailed process will producethe most advantageous results, the control of carbon profile availablevia ion implantation will enable a detailed optimization of the finaltransistor properties

A further benefit of using clusters of carbon for incorporating carbonfor stress engineering relates to the self-amorphization feature ofcluster implantation. For the appropriate stress to be generated, theincluded carbon must occupy substitution sites with the SiC latticestructure. The degree of inclusion in substitution sites depends on boththe means of incorporating the carbon and the temperature exposure ofthe material. Conventional means of incorporating carbon, whetherepitaxial or monomer implant, involve adding carbon to a crystallinestructure, while the duster carbon implant provides a self-amorphizedlayer. The amorphous layer formed by the cluster carbon implant must berecrystallized, but this is achieved automatically by the annealing ofthe dopant implants. The recrystallization process, however, promotesincorporation of the carbon into substitution sites. Such process issimilar to the incorporation of dopant atoms into substitution sites,which is well known for recrystallization processes.

Method of Incorporating SiC Stress-Engineered Lattice into CMOS ProcessFlow

In order to create stress engineered devices, the present inventioncomprises a fairly deep carbon implant, for example about 10 keV percarbon, and at a high dose, between 1E15/cm² and 5E15/cm², into theP-type deep source/drain region, prior to performing the boron or boroncluster S/D implant or SDE implant). This could be either a monomercarbon implant or a cluster carbon implant. The preferred embodimentwould comprise a cluster carbon implant. In order to avoid implantingthe carbon cluster into the polysilicon gate structure, a nitride capcan be deposited on top of the gate poly. After implanting carbon intothe P-type source/drain (S/D) region, a low-temperature anneal can beused to cause the carbon to occupy substitution sites in the Si lattice.A spike anneal of between about 600 C and 900 C, such as a 5 sec RTAtreatment, should accomplish the desired result. Data we have producedon bare Si wafers using 10 keV effective C implants using C₇H_(x) ⁺implantation at about 80 kV extraction followed by 700 C, 900 C and 1100C RTA anneals are shown in FIG. 10. The lowest temperature annealsyielded the best results, i.e., the highest strain values. After thisanneal, the CMOS structures outlined in FIGS. 12-17 can be performed tomake finished, stress-engineered devices. If a nitride cap or other maskbarrier was deposited the poly gate prior to carbon implantation; thenthe barrier would be removed before implanting the S/D structures.

Alternatively, on can simply implant carbon as shown in FIG. 11 andforgo the annealing step until the S/D implants have been performed.While there appears to be some loss of stress at higher annealtemperatures, it is not known at this time which anneal sequence willyield the highest mobility devices.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing wherein:

FIG. 1 shows a mass spectrum of B₁₈H₂₂ as produced by an ionimplantation system.

FIG. 2 shows a mass spectrum of C₁₆H₁₀ as produced by an ionimplantation system.

FIG. 3 shows Secondary Ion mass Spectrometry (SIMS) depth profiles ofboron implanted into silicon by B₁₈H_(X) ⁺, and the effect of C₁₆H_(x) ⁺co-implantation on the activated profiles.

FIG. 4 shows the as-implanted SIMS carbon profiles at 1 keV, 2 keV, 3keV, 4 keV, and 5 keV carbon implant energies, using C₁₆H_(x) ⁺.

FIG. 5 shows SIMS profiles of 300 eV boron implants, both as-implantedand annealed, after 1 keV carbon implants. The carbon profiles (beforeand after anneal) are also shown.

FIG. 6 shows junction depths for various carbon+boron implantconditions.

FIG. 7 shows the sheet resistance values associated with the junctionsproduced in FIG. 6.

FIG. 8 shows the mass spectrum of an ion beam generated from bibenzyl(also known as dibenzyl or 1,2-diphenylethane), or C₁₄H₁₄. The massspectrum shows a strong peak at 91 amu corresponding to C₇H_(x) ⁺, ionspecies, and a range of smaller peaks corresponding to C₆H_(x) ⁺,C₅H_(x) ⁺, C₄H_(x) ⁺, C₃H_(x) ⁺, and C₂H_(x) ⁺.

FIG. 9 shows the SIMS profile (carbon concentration versus depth) forC₇H₇ implants at 10 keV for three different doses (2E15, 4E15 and 8E15atoms/cm²).

FIG. 10 shows Raman spectra for C₇H₇ implant (10 keV per carbon atom) ata 2e15 dose and annealed 700° C., 900° C. & 1100° C. for 5 sec. Theshift in Raman peak for each sample was measured and was converted tostress values in Gdyne/cm².

FIG. 11 is a diagram of a CMOS fabrication sequence during formation ofcarbon doped and amorphized layers by implantation of carbon clusters.The areas corresponding to amorphized layers are shown.

FIG. 12 is a diagram of a CMOS fabrication sequence during formation ofthe NMOS drain extension.

FIG. 13 is a diagram of a CMOS fabrication sequence during formation ofthe PMOS drain extension.

FIG. 14 is a diagram of a semiconductor substrate in the process ofmanufacturing a NMOS semiconductor device; at the step of N-type drainextension implant. FIG. 15 is a diagram of a semiconductor substrate inthe process of manufacturing a NMOS semiconductor device, at the step ofthe source/drain implant.

FIG. 16 is a diagram of a semiconductor substrate in the process ofmanufacturing an PMOS semiconductor device, at the step of P-type drainextension implant.

FIG. 17 is a diagram of a semiconductor substrate in the process ofmanufacturing a PMOS semiconductor device, at the step of thesource/drain implant.

DETAILED DESCRIPTION

FIG. 1 shows a mass spectrum of B₁₈H₂₂ as produced by an ionimplantation system. A ClusterIon® source, for example, as mentionedabove, is used to generate ions which are extracted at 20 kV andtransported through an analyzer magnet. A resolving aperture at theexist of the magnet provided a modest mass resolution of M/ΔM=15; thebeam is scanned across the resolving aperture and the ion current ispassed the resolving aperture and measured by a Faraday located about 2meters from the source. The parent peak at 210 amu is composed ofB₁₈H_(X) ⁺; there is a range of retained H atoms of perhaps 10<x<22which broadens the peak. The y-axis of the plot is the beam currentmultiplied by 18 (since there are 18 boron atoms per unit charge), sothat the Faraday current was about 400 uA at mass 210. The effectiveboron implant energy is about 20 kV/20 (since the average naturalabundance boron mass is 10.8 amu and the ion mass about 210 amu)=1 keVper boron atom in the cluster. FIG. 2 shows a mass spectrum offluoranthene, C₁₆H₁₀, as produced by an ion implantation system. Theparent peak C₁₆H_(x+) is at 211 amu, and the Faraday current was about500 uA at 17 kV extraction voltage. Thus, the effective implant energyper carbon atom was about 1 keV, and the effective carbon current about8 mA. Note that the mass; effective current, and implantation energy ofthe C of FIG. 2 and B of FIG. 1 are about the same. The spectrum of FIG.2 was generated by placing solid fluoranthene in the vaporizer of an ionsource, such as the ClusterIon source, held at 100 C. The source wasinstalled in an ion implanter, for example, an Eaton NV-100 GSDimplanter. Wafers were implanted on a batch-style spinning disk toproduce the data of FIGS. 3-7.

As is known in the art, the ion source, which forms part of the ionimplanter, is used to produce ionized atoms or molecules forimplantation into a target substrate. The ion implanter, such as the onementioned above, selects the ionized molecules of interest andaccelerates those molecules into a target by way of an electric field.

FIG. 3 shows Secondary Ion Mass Spectrometry (SIMS) depth profiles ofboron implanted into silicon by B₁₈H_(X) ⁺ extracted at 6 kV (yieldingan effective per boron implant energy of 300 eV) and the effect ofC₁₆H_(x) ⁺ co-implantation on the activated profiles. The as-implantedprofile of a 5.6E13 dose of B₁₈H_(x) ⁺, i.e., a 1E15 effective borondose (labeled as B18 As-implanted) was annealed at 950 C for 5 secondsin an Axcelis Summit rapid thermal annealing system (see, for example,www.axcelis.com/products/summitXT.html) for a description of Axcelis'sRapid Thermal Annealing system The post-anneal boron profile is labeledas (B18). The effective junction depth has diffused out from about 10 nmto about 25 nm (we use a dopant concentration of 5E18 cm² as thereference point for the junction depth) due to transient enhanceddiffusion of the boron during the anneal. Other wafers were annealedwith this process which were first implanted with a 1E15 dose of either1 keV, 2 keV, 3 keV, 4 keV, or 5 keV effective carbon dose using thecarbon cluster C₁₈H_(x) ⁺. The annealed boron SIMS profiles for (B18+1keV C) and for B1.8+5 keV C) are shown in FIG. 3. The junction depthsare much shallower for these which indicates that the carbon implant hassuccessfully limited the boron diffusion. The shapes of these profilesare also quite different. While the shallowest annealed junction ofabout 15 nm (as compared to a junction depth of 25 nm without carbon)was obtained by (B18+1 keV C), a very abrupt, box-like junction wasobtained by the process (B18+5 keV C) at a junction depth of about 18nm.

FIG. 4 shows the as-implanted SIMS carbon profiles at effectiveimplantation energies of 1 keV, 2 keV, 3 keV, 4 keV, and 5 keV, using atextraction voltages of approximately 17 kV, 34 kV, 51 kV, 68 kV, and 85kV, respectively. The implantation depths correspond well to those onewould obtain using monomer C⁺ implantation at the effective implantenergies.

FIG. 5 shows SIMS profiles of 300 eV boron implants, both as-implantedand annealed, after 1 keV carbon implants. The carbon profiles (beforeand after anneal) are also shown, indicating that unlike boron, thecarbon does not diffuse or change concentration during the anneal. Theas-implanted and annealed boron profiles are similar to those shown inFIG. 3, except that the as-implanted boron profile of FIG. 5 indicatesit is free of channeling effects. This is clearly seen when compared tothe as-implanted boron profile of FIG. 3, which shows a long deep tailat concentrations below 8E17 cm². This effect is therefore solely due tothe implantation of the carbon cluster which was conducted first in thedata of FIG. 5, but not conducted prior to the boron cluster-implant ofFIG. 3. Thus, the carbon cluster implant provides two significantbenefits: (1) pre-amorphization of the silicon, reducing or eliminatingchanneling of the subsequent boron implant, and (2) diffusion controlduring anneal. These two benefits are provided even when the boronimplant does not use a cluster, but would work with monomer B as well.

FIG. 6 shows junction depths for various carbon+boron implantconditions, after anneal. As expected, the 300 eV boron junctions areshallower than the 500 eV junctions. The shallowest junctions are for acarbon implant energy of about 2 keV. There is benefit to implantingcarbon shallower rather than deeper, since the risk of generatingleakage due to the carbon should be reduced at the shallow (S/Dextension region) junction rather than at the deeper (deep S/D region)junction. Ideally, one would want the carbon to be at the same range asthe shallowest boron implant to minimize leakage. The use of clusters ofcarbon enables shallow carbon implants at higher dose than monomercarbon at the lowest implantation energies.

FIG. 7 shows the sheet resistance values associated with the junctionsproduced in FIG. 6. Since shallower junctions tend to produce highersheet resistances, the trends of FIG. 7 are different than those of FIG.6. However, the 2 keV carbon+300 eV or 500 eV B data show both areduction in junction depth and a reduction in sheet resistance,relative to the data using a 1 keV carbon implant. This indicates a realimprovement in activation when using the higher carbon energy.

FIG. 8 shows the mass spectrum of an ion beam generated from bibenzyl(also known as dibenzyl or 1,2-diphenylethane), or C₁₄H₁₄. The massspectrum shows a strong peak at about 91 amu corresponding to C₇H₇ ⁺ ionspecies. During ionization the bibenzyl molecule breaks into two C₇H₇molecules. The peak at about 182 amu corresponds to C14H14. It is alsoseen that ion beams of other carbon cluster species can be formed, suchas C₂H_(x) ⁺, C₃H_(x) ⁺, C₄H_(x) ⁺, C₅H_(x) ⁺, C₆H_(x) ⁺, and C₈H_(x) ⁺and that these species could be used to implant carbon into silicon, forvarious technological reasons, such as to implant at higher processenergy within the constraint of a given analysis magnet.

FIG. 9 shows the SIMS profile (carbon concentration versus depth) forC₇H₇ implants at 10 keV for three different doses (2E15, 4E15 and 8E15atoms/cm2). FIG. 10 shows Raman spectra for C₇H₇ implant (10 keV percarbon atom) at a 2e15 dose and annealed 700° C., 900° C. & 1100° C. for5 sec. The shift in Raman peak for each sample was measured and wereconverted to stress values in Gdyne/cm2. the values obtained show thatlower anneal temperature at 700 C gave a higher stress value whencompared to higher anneal temperatures. It is shown that significantsubstitution carbon could be achieved using this carbon molecularimplant. Formation Of N- And P-Type Shallow Junctions

An important application of this method is the use of cluster ionimplantation for the formation of N- and P-type shallow junctions aspart of a CMOS fabrication sequence. CMOS is the dominant digitalintegrated circuit technology in current use and its name denotes theformation of both N-channel and P-channel MOS transistors (ComplementaryMOS: both N and P) on the same chip. The success of CMOS is that circuitdesigners can make use of the complementary nature of the oppositetransistors to create a better circuit, specifically one that draws lessactive power than alternative technologies. It is noted that the N and Pterminology is based on Negative and Positive (N-type semiconductor hasnegative majority carriers, and vice versa); and the N-channel andP-channel transistors are duplicates of each other with the type(polarity) of each region reversed. The fabrication both types oftransistors on the same substrate requires sequentially implanting anN-type impurity and then a P-type impurity, while protecting the othertype of devices with a shielding layer of photoresist. It is noted thateach transistor type requires regions of both polarities to operatecorrectly, but the implants which form the shallow junctions are of thesame type as the transistor: N-type shallow implants into N-channeltransistors and P-type shallow implants into P-channel transistors. Anexample of this process is shown in FIGS. 12 and 13

In FIG. 11, a partially completed process is shown. The completedprocesses include the formation of the wells (N-well 81 and P-well 82),the formation of the trench isolation structure 85, the formation of thegate dielectric 84 and the deposition and patterning of the gateelectrode material 83. In addition, the appropriate photo resist maskmaterial 86 has been deposited and patterned for the formation of theNMOS transistor. Within the context of the present invention, first acluster carbon ion 88 is implanted in the unmasked regions [not 89, asimilar but different region] of the substrate in the region that willlater become the NMOS Drain Extension and NMOS Source/Drain structures,to a depth of typically 20-50 nm or just below, for example, thejunction depth of the intended device, as illustrated in FIG. 11. ACluster carbon ion implant, as described above, of 3E14 to 2E15 willamorphize a shallow silicon layer, in addition to doping the siliconwith carbon.

Once the carbon cluster ion implant is implanted, FIG. 12 illustrates amethod for forming the N-channel drain extension 89 through an N-typecluster implant 88, while FIG. 13 shows the formation of the P-channeldrain extension 90 by a P-type cluster implant 91. It is to be notedthat both. N- and P-types of transistors requires shallow junctions ofsimilar geometries, and thus having both N-type and P-type clusterimplants is advantageous for the formation of advanced CMOS structures.

An example of the application of this method is shown in FIG. 14 for thecase of forming an NMOS transistor. This figure shows semiconductorsubstrate 41 which has undergone some of the front-end process steps ofmanufacturing a semiconductor device. For example, the structureconsists of a N-type semiconductor substrate 41 processed through theP-well 43, trench isolation 42, and gate stack formation 44, 45 steps.An exemplary process for forming the gate stack, P-well and trenchisolation is disclosed in International Publication No. WO 2004/003973A2, published on Jan. 8, 2004, entitled: “An Ion Implantation Device anda Method of semiconductor Manufacturing By the Implantation of BoronHydride Cluster Ions, hereby incorporated by reference.

An arsenic implant has historically been used to form the NMOS SDEstructure, and since As exhibits slow diffusion in silicon, so noadditional means are necessary to form the appropriate structure.However, there are issues which may preclude the use of As for the SDEin advanced technologies. For example, phosphorus can be activated inhigher concentrations than arsenic, since silicon has a higher solidsolubility for phosphorus than for arsenic. Thus, phosphorus may yieldlower resistivity S/D regions than arsenic, an important feature sinceresistivity tends to increase as devices become shallower (smaller), asin advanced devices. In these cases, phosphorus may become a viablealternative, but unlike arsenic, phosphorus exhibits fast diffusion insilicon and so diffusion inhibition by carbon becomes an attractiveprocess. The P-well 43 forms a junction with the N: type substrate 41that provides junction isolation for the transistors in the well 43. Thetrench isolation 42 provides lateral dielectric isolation between the N-and P-wells (i.e., in the overall CMOS structure). The gate stack isconstructed, with a gate oxide layer 44 and a polysilicon gate electrode45, patterned to form a transistor gate stack. A photoresist 46 isapplied and patterned such that the area for NMOS transistors isexposed, but other areas of the substrate 41 are shielded. After thephotoresist 46 is applied, the substrate 41 is ready for the drainextension implant, which is the shallowest doping layer required by thedevice fabrication process.

A typical process requirement for leading-edge devices of the 0.10 μmtechnology node is an arsenic implant energy of between 1 keV and 2 keV,and an arsenic dose of 5×10¹⁴ cm⁻². The cluster ion beam 47, As₄H_(x) ⁺in this case, is directed at the semiconductor substrate, typically suchthat the direction of propagation of the ion beam is normal to thesubstrate, to avoid shadowing by the gate stack. The energy of theAs₄H_(x) ⁺ cluster should be four times the desired As⁺ implant energy,e.g., between 4 keV and 8 keV. The clusters dissociate upon impact withthe substrate, and the dopant atoms come to rest in a shallow layer nearthe surface of the semiconductor substrate, which forms the drainextension region 48. It should be noted that the same implant enters thesurface layer of the gate electrode 49, providing additional doping forthe gate electrode. The process described in FIG. 14 is thus oneimportant application of the proposed invention.

A further example of the application of this method is shown in FIG. 15:the formation of the deep source/drain regions. This figure shows thesemiconductor substrate 41 of FIG. 14 after execution of furtherprocesses steps in the fabrication of a semiconductor device. Theadditional process steps include the formation of a pad oxide 51 and theformation of spacers 52 on the sidewalls of the gate stack. Reiteratingthe process steps described in FIG. 14 but identified in FIG. 15 forclarity, The P-well 43 forms a junction with the N-type substrate 41that provides junction isolation for the transistors in the well 43. Thetrench isolation 42 provides lateral dielectric isolation between the N-and P-wells (i.e., in the overall CMOS structure). The gate stack isconstructed, with a gate oxide layer 44 and a polysilicon gate electrode45, patterned to form a transistor gate stack. A photoresist 46 isapplied and patterned such that the area for NMOS transistors isexposed, but other areas of the substrate 41 are shielded. After thephotoresist 46 is applied. The cluster ion beam 54, As₄H_(x) ⁺ in thiscase, although a phosphorus implant of similar dose may be used instead,is directed at the semiconductor substrate, typically such that thedirection of propagation of the ion beam is normal to the substrate, toavoid shadowing by the gate stack. The dopant atoms come to rest in ashallow layer near the surface of the semiconductor substrate, whichforms the drain extension region 48. It should be noted that the sameimplant enters the surface layer of the gate electrode 49, providingadditional doping for the gate electrode. The pad oxide 51 is a thinlayer of oxide (silicon dioxide) used to protect the exposed substrateareas, the top of the gate electrode 49 and the potentially exposed gatedielectric edge. The pad oxide 51 is typically thermally grown to athickness of 5-10 nm. The spacer 52, on the other hand, is a region ofdielectric, either silicon dioxide, silicon nitride, or a combination ofthese, which resides on the side of the gate stack and serves toinsulate the gate electrode. It also serves as an alignment guide forthe source/drain implant (e.g., 54), which must be spaced back from thegate edge for the transistor to operate properly. The spacers 52 areformed by the deposition of silicon dioxide and/or silicon nitridelayers which are then plasma etched in a way to leave a residual layeron the side of the gate stack while clearing the dielectrics from thesource/drain region. After etching the spacers 52, a photoresist layer53 is applied and patterned to expose the transistor to be implanted, anNMOS transistor in this example. Next, the ion implant to form thesource and drain regions 55 is performed. Since this implant requires ahigh dose at low energy, it is an appropriate application of theproposed cluster implantation method. Typical implant parameters for the0.13 um technology node are approximately 6 keV per arsenic atom (54) atan arsenic dose of 5×10¹⁵ cm⁻², so it requires a 24 keV, 1.25×10¹⁵ cm⁻²As₄H_(x) ⁺ implant, a 12 keV, 2.5×10¹⁵ cm⁻² As₂H_(x) ⁺ implant, or a 6keV, 5×10¹⁵ cm⁻² As⁺ implant.

As shown in FIG. 14, the source and drain regions 55 are formed by thisimplant. These regions provide a high conductivity connection betweenthe circuit interconnects (to be formed later in the process) and theintrinsic transistor defined by the drain extension 48 in conjunctionwith the channel region 56 and the gate stack 44, 45. The gate electrode45 can be exposed to this implant (as shown), and if so, thesource/drain implant provides the primary doping source for the gateelectrode. This is shown in FIG. 15 as the poly doping layer 57.

The detailed diagrams showing the formation of the PMOS drain extension148 and PMOS source and drain regions 155 are shown in FIGS. 16 and 17,respectively. The structures and processes are the same as in FIGS. 15and 16 with the dopant types reversed. Thus, FIG. 16 shows semiconductorsubstrate 141 which has undergone some of the front-end process steps ofmanufacturing a semiconductor device. For example, the structureconsists of a P-type semiconductor substrate 141 processed through theN-well 143, trench isolation 142, and gate stack formation 144, 145steps. A boron implant has historically been used to form the PMOS SDEstructure, however in the present invention a boron cluster ion such asB₁₈H_(x) ⁺ is used.

The N-well 143 forms a junction with the P-type substrate 141 thatprovides junction isolation for the transistors in the well 143. Thetrench isolation 142 provides lateral dielectric isolation between theN- and P-wells (i.e., in the overall CMOS structure). The gate stack isconstructed, with a gate oxide layer 144 and a polysilicon gateelectrode 145, patterned to form a transistor gate stack. A photoresist146 is applied and patterned such that the area for PMOS transistors isexposed, but other areas of the substrate 141 are shielded. After thephotoresist 146 is applied, the substrate 141 is ready for the drainextension implant, which is the shallowest doping layer required by thedevice fabrication process. A typical process requirement forleading-edge devices of the 0.10 μm technology node is a boron implantenergy of between 0.5 keV and 1 keV, and an boron dose of 5×10¹⁴ cm⁻².The cluster ion beam 147, B₁₈H_(x) ⁺ in this case, is directed at thesemiconductor substrate, typically such that the direction ofpropagation of the ion beam is normal to the substrate, to avoidshadowing by the gate stack. The energy of the B₁₈H_(x) ⁺ cluster shouldbe 20 times the desired B⁺ implant energy, e.g., between 10 keV and 20keV, and the B₁₈H_(x) ⁺ dose should be one-eighteenth of the boron dose,about 2.8E13. The clusters dissociate upon impact with the substrate,and the dopant atoms come to rest in a shallow layer near the surface ofthe semiconductor substrate, which forms the drain extension region 148.It should be noted that the same implant enters the surface layer of thegate electrode 149, providing additional doping for the gate electrode.The process described in FIG. 16 is thus one important application ofthe proposed invention.

A further example of the application of this method is shown in FIG. 17:the formation of the deep source/drain regions. This figure shows thesemiconductor substrate 141 of FIG. 16 after execution of furtherprocesses steps in the fabrication of a semiconductor device. Theadditional process steps include the formation of a pad oxide 151 andthe formation of spacers 152 on the sidewalls of the gate stack.Reiterating the process steps described in FIG. 16 but identified inFIG. 17 for clarity, The N-well 143 forms a junction with the P-typesubstrate 141 that provides junction isolation for the transistors inthe well 143. The trench isolation 142 provides lateral dielectricisolation between the N- and P-wells (i.e., in the overall CMOSstructure). The gate stack is constructed, with a gate oxide layer 144and a polysilicon gate electrode 145, patterned to form a transistorgate stack. A photoresist 146 is applied and patterned such that thearea for PMOS transistors is exposed, but other areas of the substrate141 are shielded. After the photoresist 146 is applied. The cluster ionbeam 154, B₁₈H_(x) ⁺ in this case is directed at the semiconductorsubstrate, typically such that the direction of propagation of the ionbeam is normal to the substrate, to avoid shadowing by the gate stack.The dopant atoms come to rest in a shallow layer near the surface of thesemiconductor substrate, which forms: the drain extension region 148. Itshould be noted that the same implant enters the surface layer of thegate electrode 149, providing additional doping for the gate electrode.The pad oxide 151 is a thin layer of oxide (silicon dioxide) used toprotect the exposed substrate areas, the top of the gate electrode 149and the potentially exposed gate dielectric edge. The pad oxide 151 istypically thermally grown to a thickness of 5-10 nm. The spacer 152, onthe other hand, is a region of dielectric, either silicon dioxide,silicon nitride, or a combination of these, which resides on the side ofthe gate stack and serves to insulate the gate electrode. It also servesas an alignment guide for the source/drain implant (e.g., 154), whichmust be spaced back from the gate edge for the transistor to operateproperly. The spacers 152 are formed by the deposition of silicondioxide and/or silicon nitride layers which are then plasma etched in away to leave a residual layer on the side of the gate stack whileclearing the dielectrics from the source/drain region.

After etching the spacers 52, a photoresist layer 53 is applied andpatterned to expose the transistor to be implanted, a PMOS transistor inthis example. Next, the ion implant to form the source and drain regions155 is performed. Since this implant requires a high dose at low energy,it is an appropriate application of the proposed cluster implantationmethod. Typical implant parameters for the 0.10 um technology node areapproximately 4 keV per boron atom (154) at a boron dose of 5×10¹⁵ cm⁻²,so it requires an 80 keV, 2.8×10¹⁴ cm⁻² B₁₈H_(x) ⁺ implant, or a 4 keV,5×10¹⁵ cm⁻² B⁺ implant. These regions provide a high conductivityconnection between the circuit interconnects (to be formed later in theprocess) and the intrinsic transistor defined by the drain extension 148in conjunction with the channel region 156 and, the gate stack 144, 145.The gate electrode 145 can be exposed to this implant (as shown), and ifso, the source/drain implant provides the primary doping source for thegate electrode. This is shown in FIG. 17 as the poly doping layer 157.

In general, ion implantation alone is not sufficient for the formationof an effective semiconductor junction: a heat treatment is necessary toelectrically activate the implanted dopants. After implantation, thesemiconductor substrate's crystal structure is heavily damaged(substrate atoms are moved out of crystal lattice positions), and theimplanted dopants are only weakly bound to the substrate atoms, so thatthe implanted layer has poor electrical properties. A heat treatment, oranneal, at high temperature (greater than 900 C) is typically performedto repair the semiconductor crystal structure, and to position thedopant atoms substitution ally, i.e., in the position of one of thesubstrate atoms in the crystal structure. This substitution allows thedopant to bond with the substrate atoms and become electrically active;that is, to change the conductivity of the semiconductor layer. Thisheat treatment works against the formation of shallow junctions,however, because diffusion of the implanted dopant occurs during theheat treatment. Boron diffusion during heat treatment, in fact, is thelimiting factor in achieving USJ's in the sub-0.1 micron regime.Advanced processes have been developed for this heat treatment tominimize the diffusion of the shallow implanted dopants, such as the“spike anneal”. The spike anneal is a rapid thermal process wherein theresidence time at the highest temperature approaches zero: thetemperature ramps up and down as fast as possible. In this way, the hightemperatures necessary to activate the implanted dopant are reachedwhile the diffusion of the implanted dopants is minimized. It isanticipated that such advanced heat treatments would be utilized inconjunction with the present invention to maximize its benefits in thefabrication of the completed semiconductor device. In particular, theimplanted carbon reduces the transient enhanced diffusion of dopantduring the thermal activation process.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than is specifically described above

1. A method of implanting ions comprising the steps of: (a) producing avolume of gas phase molecules of bibenzyl, C₁₄H₁₄ molecules; (b)ionizing the C₁₄H₁₄ molecules to produce an ion beam of C₇H_(y) ions;and (c) accelerating the C₇H_(y) ions by an electric field into atarget.
 2. A method of implanting ions comprising the steps of: (a)producing a volume of gas phase molecules of material of the form C₁₄H₁₄molecules; (b) ionizing the C₁₄H₁₄ molecules to form C₇H_(x) ⁺ ions; and(c) accelerating the C₇H_(x) ⁺-ions by an electric field into a target.3. A method of implanting ions comprising the steps of: (a) producing avolume of gas phase molecules of material of the form C₁₄H₁₄; (b)ionizing the C₁₄H₁₄ molecules to C_(n)H_(x) ⁺ and C_(n-1)H_(x) ⁺ ions,where n is an integer and 3<n<8; and (c) accelerating the ionizedmolecules by an electric field into a target.
 4. A method of implantingions comprising the steps of: (a) producing a volume of gas phasemolecules of material of the form C_(n)H_(x) molecules, wherein n and xare integers, and 2<=n and x=0, 1 (b) ionizing the C_(n)H_(x) moleculesto form C_(z)H_(y) ⁺ ions, where z and y are integers and 2<z<n and y>0;and (c) accelerating the ionized molecules by an electric field into atarget.